A time of flight mass spectrometer

A time of flight mass spectrometer including a first electrode 3 and a second electrode 4. The ion source is configured to apply voltages to the first and second electrodes to produce an electric field in a region between the first and second electrodes. A shield 10 is formed on the first electrode and/or second electrode. The shield is configured to inhibit an electric field formed between edges of the first and second electrodes from penetrating into the region between the first and second electrodes when the mass spectrometer is in use. Inhibiting this side field has been found to result in reduced drop in intensity depending upon where the sample is positioned. The shield may be a raised element, or may surround an axis extending between electrodes. The electrode may have a second shield which surrounds an aperture. The two electrodes may be included in the ion source of the mass spectrometer with the electrodes being used to produce an extraction electric field between them so as to extract the ions when in use.